1. Field of the Invention
The present invention relates generally to methods of power control for mobile terminals, and in particular to methods for adjusting power control step size for a mobile terminal.
2. Discussion of the Related Art
Numerous access schemes exist to allow multiple users to share a communication medium. One such access scheme is known as code division multiple access (CDMA). In CDMA systems, multiple users share the same carrier frequency and may transmit simultaneously.
The current standard for CDMA systems is contained in specifications published by the Telecommunications Industry Association and Electronics Industry Association (TIA/EIA), and include IS-95A, IS-95B, and other CDMA-type protocols. New standards for wideband CDMA have been developed and offer significant performance improvements compared to the current CDMA standard. One such standard is known as cdma2000. cdma2000 is a wideband, spread-spectrum radio interface that uses CDMA technology to satisfy the needs of third generation wireless communication systems. Several enhancements of the cdma2000 standard have been developed to facilitate the gradual evolution of third generation wireless communication systems. The cdma2000 variant known as 1xEV-DO is being developed to provide high-speed packet data services as an overlay to existing circuit-switched networks.
One functional aspect of a CDMA system relates to power control. Power control is used on the reverse link in CDMA systems to control the power of signals received at each base station from the mobile terminals. One purpose of power control is to assure that each mobile terminal served by a particular base station provides approximately the same signal level to the receiver at that sector. In CDMA systems, system capacity is maximized if the transmit power level of each mobile terminal is controlled so that its signals arrive at the base station receiver with the minimum required signal-to-noise ratio (SNR) or signal-to-interference ratio (SIR). The target value for the received power level is the minimum level possible that allows the link to meet the predetermined performance objectives.
As the mobile terminal moves around within the network, the channel conditions change continuously due to fast and slow fading, shadowing, number of users, external interference, and other factors. Conventional power control algorithms dynamically control the transmitted power on the reverse link to maintain the minimum SNR or SIR at the base station. Both open loop and closed loop power control are typically used on the reverse link. In open loop control, the mobile terminal monitors the received signal strength on the forward link and varies its transmit power inversely in proportion to the measured signal strength. Fast power control is provided by the closed loop power control mechanism. In closed loop power control, the base station measures the strength of the received signal from the mobile terminal and sends power control commands to the mobile terminal requesting the mobile terminal to either increase or decrease its transmit power.
Current standards permit adjusting of transmission power of a mobile terminal, for example, according to defined step sizes. However, selection or adjustment of the step size is typically accomplished using upper layer messaging, which is slow to react to quickly changing channel conditions.